Conventional rain sensors include, for example, that of German Patent No. 197 01 258, in which the radiation of a transmitter is coupled into the window of a motor vehicle by a holographically embodied diffractive element, totally reflected at the outer side of the window, and directed or focused onto a receiver by a further diffractive element. In such sensors, the diffractive elements are embodied as volume phase holograms that are introduced into films made of photopolymers.
Such films are complex to produce, expensive to manufacture, and problematic in terms of their environmental properties.